What is “coma” and how does it affect my telescope?
Coma is an optical phenomenon that results when off-axis light rays are reflected by a parabolic mirror and brought to a focus. The effect is star images away from the telescopic center of the field of view are not round, but have a somewhat triangular shape. Coma generally becomes apparent at focal ratios below f/6, with the effects increasingly pronounced at focal ratios of f/4. Meade Schmidt-Newtonian reflectors are designed to reduce the effects of coma by one-half, thereby allowing the benefits of wide-field viewing and fast photographic speeds with very little coma visible.
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